Optical element containing an optical spacer

ABSTRACT

Disclosed is an optical element containing a rough surface having a roughness average equal to at least 5 micrometers wherein the rough surface contains at least two roughness populations in which the roughness average of the at least two populations varies by at least 8 micrometers. The invention provides an optical element containing an integral optical spacer while simultaneously providing scratch and impact resistance.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is one of a group of five applications co-filedunder Attorney Docket Nos. 83948/AEK, 84008/AEK, 84301/AEK, 84393/AEK,and 84407/AEK, the contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

[0002] The invention relates to an optical element suitable fordiffusion of specular light and the control of the light diffusionintensity containing a rough surface having a roughness average equal toat least 5 micrometers wherein the rough surface contains at least tworoughness populations in which the roughness average of the at least twopopulations varies by at least 8 micrometers.

BACKGROUND OF THE INVENTION

[0003] Optical structures that scatter or diffuse light generallyfunction in one of two ways: (a) as a surface diffuser utilizing surfaceroughness to refract or scatter light in a number of directions; or (b)as a bulk diffuser having flat surfaces and embedded light-scatteringelements.

[0004] A diffuser of the former kind is normally utilized with its roughsurface exposed to air, affording the largest possible difference inindex of refraction between the material of the diffuser and thesurrounding medium and, consequently, the largest angular spread forincident light. However, some prior art light diffusers of this typesuffer from a major drawback: the need for air contact. The requirementthat the rough surface must be in contact with air to operate properlymay result in lower efficiency. If the input and output surfaces of thediffuser are both embedded inside another material, such as an adhesivefor example, the light-dispersing ability of the diffuser may be reducedto an undesirable level.

[0005] In one version of the second type of diffuser, the bulk diffuser,small particles or spheres of a second refractive index are embeddedwithin the primary material of the diffuser. In another version of thebulk diffuser, the refractive index of the material of the diffuservaries across the diffuser body, thus causing light passing through thematerial to be refracted or scattered at different points. Bulkdiffusers also present some practical problems. If a high angular outputdistribution is sought, the diffuser will be generally thicker than asurface diffuser having the same optical scattering power. If howeverthe bulk diffuser is made thin, a desirable property for mostapplications, the scattering ability of the diffuser may be too low.

[0006] Despite the foregoing difficulties, there are applications wherea surface diffuser may be desirable, where the bulk type of diffuserwould not be appropriate. For example, the surface diffuser can beapplied to an existing film or substrate thus eliminating the need tofor a separate film. In the case of light management in a LCD, thisincreases efficiency by removing an interface (which causes reflectionand lost light).

[0007] Prior art optical elements which include light diffusers, lightdirectors, light guides, brightness enhancement films and polarizingfilms typical comprise a repeating ordered geometrical pattern or randomgeometrical pattern. The geometrical patterns typically have a singlesize distribution in order to accomplish the intended optical function.An example is a brightness enhancement film for LC displays utilizingprecise micro prisms. The micro prism geometry has a single sizedistribution across the sheet and when utilized with a polarizing sheet,the top of the micro prisms are in contact with the polarizing sheet.When these prior art optical elements are used as a system, as is thecase in a liquid crystal display, the optical elements are typically inoptical contact. The focal length of the prior art optical elements, incombination with other optical elements, typically comprise thethickness of the optical element.

[0008] Prior art optical spacer materials typically comprise thin,transparent polymer sheets to provide optical spacing between twooptical components. Optical spacer materials are utilized to change thefocal length of an optical component or to provide protection betweentwo optical components. It would be desirable for an optical componentto contain an integral optical spacer.

[0009] U.S. Pat. No. 6,270697 (Meyers et al.), blur films are used totransmitted infrared energy of a specific waveband using a repeatingpattern of peak-and-valley features. While this does diffuse visiblelight, the periodic nature of the features is unacceptable for abacklight LC device because the pattern can be seen through the displaydevice.

[0010] U.S. Pat. No. 6,266,476 (Shie et al.) discloses a microstructureon the surface of a polymer sheet for the diffusion of light. Themicrostructures are created by molding Fresnel lenses on the surface ofa substrate to control the direction of light output from a light sourceso as to shape the light output into a desired distribution, pattern orenvelope. While the materials disclosed in U.S. Pat. No. 6,266,476 shapeand collimate light and therefore are not efficient diffusers of lightparticularly for liquid crystal display devices. Further, themicro-structures are of a single size distribution.

[0011] It is known to produce transparent polymeric film having a resincoated on one surface thereof with the resin having a surface texture.This kind of transparent polymeric film is made by a thermoplasticembossing process in which raw (uncoated) transparent polymeric film iscoated with a molten resin, such as polyethylene. The transparentpolymeric film with the molten resin thereon is brought into contactwith a chill roller having a surface pattern. Chilled water is pumpedthrough the roller to extract heat from the resin, causing it tosolidify and adhere to the transparent polymeric film. During thisprocess the surface texture on the chill roller's surface is embossedinto the resin coated transparent polymeric film. Thus, the surfacepattern on the chill roller is critical to the surface produced in theresin on the coated transparent polymeric film.

[0012] One known prior process for preparing chill rollers involvescreating a main surface pattern using a mechanical engraving process.The engraving process has many limitations including misalignmentcausing tool lines in the surface, high price, and lengthy processing.Accordingly, it is desirable to not use mechanical engraving tomanufacture chill rollers.

[0013] The U.S. Pat. No. 6,285,001 (Fleming et al) relates to anexposure process using excimer laser ablation of substrates to improvethe uniformity of repeating microstructures on an ablated substrate orto create three-dimensional microstructures on an ablated substrate.This method is difficult to apply to create a master chill roll tomanufacture complex random three-dimensional structures and is also costprohibitive.

[0014] In U.S. Pat. No. 6,124,974 (Burger) the substrates are made withlithographic processes. This lithography process is repeated forsuccessive photomasks to generate a three-dimensional relief structurecorresponding to the desired lenslet. This procedure to form a master tocreate three-dimensional features into a plastic film is time consumingand cost prohibitive.

[0015] There remains a need for an integral optical spacer for opticalcomponents to provide a variable focal length and scratch and impactresistance between two optical components.

SUMMARY OF THE INVENTION

[0016] The invention provides an optical element containing a roughsurface having a roughness average equal to at least 5 micrometerswherein the rough surface contains at least two roughness populations inwhich the roughness average of the at least two populations varies by atleast 8 micrometers. The invention also provides a back lighted imagingmedia, a liquid crystal display component and device, and method ofmaking them.

[0017] The invention provides an optical element containing an integraloptical spacer while simultaneously providing scratch and impactresistance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 illustrates a cross section of a complex lens formed on atransparent base material containing an optical spacer suitable for usein a liquid crystal display device.

[0019]FIG. 2 illustrates a liquid crystal display device with a complexlens light diffuser containing an optical spacer.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The invention has numerous advantages over prior practices in theart. The invention provides an optical element such as a light diffuser,light guide or a focusing film with an integral optical spacer. Anoptical spacer provides a method to control the focal length of anoptical component utilized in an optical system such as an LC device, byproviding a precision spacing between each of the optical components. Byproviding an integral optical spacer on the optical element of theinvention, the spacing in an optical system can be carefully controlled.Further, by providing an optical spacer, the optical element of theinvention provides scratch resistance and impact resistance as thefunctional surface of the optical element is spaced from the otheroptical components in a optical system. The integral optical spacer ofthe invention also provides a specified air gap for optical elementsthat require an air gap for light diffusion, for example. By providing aspecified air gap, the efficiency of light diffusers is improved as allof the diffusion geometry is in contact with air.

[0021] By providing an optical spacer of the invention, the opticalelement is protected from handling damage or damage during assembly ofoptical components. The optical spacer can also be used to provideprotection to the optical element against fingerprints and scratchingthat is typically encountered on the outermost layer in a visibledisplay such as a cell phone, projection television or automobiledisplay panel. Scratches and fingerprints reduce the optical utility ofsuch devices.

[0022] The optical spacer of the invention is integral to the opticalelements avoiding the need for a transparent spacer sheet. Further,since the optical spacer is integral to the optical element, reflectionand absorption losses through the prior art transparent sheet, whichtypically range from 2 to 5% loss in transmitted light energy areavoided. The invention also provides optical spacer that have a specificgeometry to provide both optical spacing and optical function such aslight shaping or light directing. Additionally, the elimination of atransparent optical spacer, avoids unwanted interference patterns whichform when transmitted light passes through two partially reflectivesurfaces. The unwanted interference patterns are unwanted as they formcolored rings visible in LC devices.

[0023] In a preferred form, the invention provides diffusion of specularlight sources that are commonly used in rear projection display devicessuch as liquid crystal display devices. Further, the invention, whileproviding diffusion to the light sources, has a high light transmissionrate. A high transmission rate for light diffusers is particularlyimportant for liquid crystal display devices as a high transmissionvalue allows the liquid crystal display to be brighter, or holding thelevel of brightness the same allows the power consumption for the backlight to be reduced, therefore extending the lifetime of battery poweredliquid crystal devices that are common for note book computers. Thesurface lenslet structure polymer layer of the invention can be easilychanged to achieve the desired diffusion and light transmissionrequirements for many liquid crystal devices thus allowing the inventionmaterials to be responsive to the rapidly changing product requirementsin the liquid crystal display market. These and other advantages will beapparent from the detailed description below.

[0024] The term “LCD” means any rear projection display device thatutilizes liquid crystals to form the image. The term “diffuser” meansany material that is able to diffuse specular light (light with aprimary direction) to a diffuse light (light with random lightdirection). The term “light” means visible light. The term “diffuselight transmission” means the percent diffusely transmitted light at 500nm as compared to the total amount of light at 500 nm of the lightsource. The term “total light transmission” means the percentage lighttransmitted through the sample at 500 nm as compared to the total amountof light at 500 nm of the light source. This includes both spectral anddiffuse transmission of light. The term “diffuse light transmissionefficiency” means the ratio of % diffuse transmitted light at 500 nm to% total transmitted light at 500 nm multiplied by a factor of 100. Theterm “polymeric film” means a film comprising polymers. The term“polymer” means homo- and co-polymers. The term “average”, with respectto lens size and frequency, means the arithmetic mean over the entirefilm surface area.

[0025] “Transparent” means a film with total light transmission of 50%or greater at 500 nm. “In any direction”, with respect to lensletarrangement on a film, means any direction in the x and y direction inthe plane of the film. The term “pattern” means any predeterminedarrangement of lenses whether regular or random.

[0026] An optical spacer generally is a transparent material such asglass or polymer that provides a specified space between two opticalcomponents. An example of an optical spacer is a light diffusercomprising a rough surface using the index of refraction differencebetween the air and the light diffuser to provide light diffusion. Ifthe air is eliminated between the diffuser, the light diffuser willloose efficiency. In an optical system requiring a light diffuser, theair gap, that is the space between the light diffuser and the nextoptical component is critical for the performance of the light diffuser.

[0027] An integral geometric optical spacer is a geometrical shape thathas a height greater than the functional optical surface, such as alight director or a light diffuser and yet is part of the surface. Forthe invention materials, the integral geometric optical spacer is formedsimultaneously with the functional optical element and thus is integralto the optical element.

[0028] Roughness average is the arithmetic average height calculatedover the entire measured array. The arithmetic mean is the mean of theabsolute values of the surface features from the mean plane. Themeasured array typically consists of 10 mm and the units are expressedin micrometers. Roughness average can be measured by contact stylus orlaser methods.

[0029] Optical components are components that have optical utility suchas light diffusion, light direction, light guiding, color filters,polarizing films and the like, which can be used in combination with theoptical element of the invention. An example is a simple system designedto diffuse specular light, accomplished by a light diffuser and a lightdirection film, accomplished by a prism film. In this example, the lightdiffuser film and the light direction film are considered opticalcomponents.

[0030] Better control and management of the back light are drivingtechnological advances for liquid crystal displays (LCD). LCD screensand other electronic soft display media are back lit primarily withspecular (highly directional) fluorescent tubes. Diffusion films areused to distribute the light evenly across the entire display area andchange the light from specular to diffuse. Light exiting the liquidcrystal section of the display stack leaves as a narrow column and mustbe redispersed. Diffusers are used in this section of the display toselectively spread the light out horizontally for an enhanced viewingangle.

[0031] Diffusion is achieved by light scattering as it passes thoughmaterials with varying indexes of refraction. This scattering produces adiffusing medium for light energy. There is an inverse relationshipbetween transmittance of light and diffusion and the optimum combinationof these two parameters must be found for each application.

[0032] The back diffuser is placed directly in front of the light sourceand is used to even out the light throughout the display by changingspecular light into diffuse light. The diffusion film is made up of aplurality of lenslets on a web material to broaden and diffuse theincoming light. Prior art methods for diffusing LCD back light includelayering polymer films with different indexes of refraction, microvoidedpolymer film, or coating the film with matte resins or beads. The roleof the front diffuser is to broaden the light coming out of the liquidcrystal (LC) with directional selectivity. The light is compressed intoa tight beam to enter the LC for highest efficient and when it exits itcomes out as a narrow column of light. The diffuser uses opticalstructures to spread the light selectively. Most companies formelliptical micro-lens to selectively stretch the light along one axis.Elliptically shaped polymers in a polymer matrix and surfacemicro-lenses formed by chemical or physical means also achieve thisdirectionality. The diffusion film of the present invention can beproduced by using a conventional film-manufacturing facility in highproductivity.

[0033] The polymeric diffusion film has a textured surface on at leastone side, in the form of a plurality of random microlenses, or lenslets.The term “lenslet” means a small lens, but for the purposes of thepresent discussion, the terms lens and lenslet may be taken to be thesame. The lenslets overlap to form complex lenses. “Complex lenses”means a major lens having on the surface thereof multiple minor lenses.“Major lenses” mean larger lenslets in which the minor lenses are formedrandomly on top of. “Minor lenses” mean lenses smaller than the majorlenses that are formed on the major lens. The plurality of lenses of alldifferent sizes and shapes are formed on top of one another to create acomplex lens feature resembling a cauliflower. The lenslets and complexlenses formed by the lenslets can be concave into the transparentpolymeric film or convex out of the transparent polymeric film. The term“concave” means curved like the surface of a sphere with the exteriorsurface of the sphere closest to the surface of the film. The term“convex” means curved like the surface of a sphere with the interiorsurface of the sphere closest to the surface of the film. The term “topsurface” means the surface of the film farther from the light source.The term “bottom surface” means the surface of the film closer to thelight source.

[0034] One embodiment of the present invention could be likened to themoon's cratered surface. Asteroids that hit the moon form craters apartfrom other craters, that overlap a piece of another crater, that formwithin another crater, or that engulf another crater. As more cratersare carved, the surface of the moon becomes a complexity of depressionslike the complexity of lenses formed in the transparent polymeric film.

[0035] The surface of each lenslet is a locally spherical segment, whichacts as a miniature lens to alter the ray path of energy passing throughthe lens. The shape of each lenslet is “semi-spherical” meaning that thesurface of each lenslet is a sector of a sphere, but not necessarily ahemisphere. Its curved surface has a radius of curvature as measuredrelative to a first axis (x) parallel to the transparent polymeric filmand a radius of curvature relative to second axis (y) parallel to thetransparent polymeric film and orthogonal to the first axis (x). Thelenses in an array film need not have equal dimensions in the x and ydirections. The dimensions of the lenses, for example length in the x ory direction, are generally significantly smaller than a length or widthof the film. “Height/Diameter ratio” means the ratio of the height ofthe complex lens to the diameter of the complex lens. “Diameter” meansthe largest dimension of the complex lenses in the x and y plane. Thevalue of the height/diameter ratio is one of the main causes of theamount of light spreading, or diffusion that each complex lens creates.A small height/diameter ratio indicates that the diameter is muchgreater than the height of the lens creating a flatter, wider complexlens. A larger height/diameter value indicates a taller, skinner complexlens. The complex lenses may differ in size, shape, offset from opticalaxis, and focal length.

[0036] The curvature, depth, size, spacing, materials of construction(which determines the basic refractive indices of the polymer film andthe substrate), and positioning of the lenslets determine the degree ofdiffusion, and these parameters are established during manufactureaccording to the invention.

[0037] The divergence of light through the lens may be termed“asymmetric”, which means that the divergence in the horizontaldirection is different from the divergence in the vertical direction.The divergence curve is asymmetric, or that the direction of the peaklight transmission is not along the direction θ=0°, but is in adirection non-normal to the surface. There are least three approachesavailable for making the light disperse asymmetrically from a lensletdiffusion film, namely, changing the dimension of the lenses in onedirection relative to an orthogonal direction, off-setting the opticalaxis of the lens from the center of the lens, and using an astigmaticlens.

[0038] The result of using a diffusion film having lenses whose opticalaxes are off-set from the center of the respective lenses results indispersing light from the film in an asymmetric manner. It will beappreciated, however, that the lens surface may be formed so that theoptical axis is off-set from the center of the lens in both the x and ydirections.

[0039] The lenslet structure can be manufactured on the opposite sidesof the substrate. The lenslet structures on either side of the supportcan vary in curvature, depth, size, spacing, and positioning of thelenslets.

[0040] An optical element containing an optical spacer provides a airgap, specified focal length and scratch and impact resistance. Anoptical element containing a rough surface having a roughness averageequal to at least 5 micrometers wherein the rough surface contains atleast two roughness populations in which the at least two populationsvary by at least 8 micrometers is preferred. The optical element withtwo roughness populations provides at least one functional opticalsurface such as a light diffuser or light guide and another populationthat provides spacing in an optical system consisting of more than oneoptical component. By providing at least one roughness population thatis higher than the others, the higher population provides the opticalcontact with other optical components while the other roughnesspopulation provides the optical utility such as light direction or lightdiffusion. Further, the two populations preferably vary by at least 8micrometers because spacing less than 5 micrometers can result inunwanted light interference patterns.

[0041] Precision control of the air gap between the optical element ofthe invention and other optical components can greatly improve theefficiency and the variability of the optical element in an opticalsystem. An example is visible light diffusion films containing anintegral optical spacer. By providing an optical spacer between asurface diffuser and other optical components, the spread of thediffused light into other optical components can be specified andcontrolled by the height of the optical spacer compared to the lightdiffusing element. A specular light source, such as a laser, can besurface diffused into a narrow cone without the use of a spacer and canbe surfaced diffused into a broad cone using a spacer that is 5 to 20times larger than the diffusion element. A narrow light diffusion conewill tend to provide narrow viewing angle in an LC device while a broadcone will provide a larger viewing angle. Both narrow cone diffusers andbroad cone diffusers have utility depending on the light diffusionapplication.

[0042] An optical element that comprises the at least two roughnesspopulations applied to both the front and back side of the sheet ispreferred. By applying the at least two roughness populations to bothsides, the optical spacing feature is present on both sides. A two-sidedoptical spacer provides for spacing when the optical element iscontained between two additional optical components such as a polarizingsheet and a light diffuser.

[0043] In a preferred embodiment, the at least two roughness populationsvary by at least 250 micrometers. By providing a optical spacing of atleast 250 micrometers, the optical element of the invention has asufficient air gap for light diffusion, light guiding or light directingand has a sufficient gap for scratch and impact resistance. In a morepreferred embodiment of the invention, the at least two roughnesspopulations vary by at least 75 micrometers. An optical spacing of atleast 75 micrometers has been shown to provide a sufficient air gap forlight diffusion, light directing and light guiding and provide asufficient spacing for the focal length of directing lenses.

[0044] In a preferred embodiment of the invention, the at least tworoughness populations are ordered. By providing an ordered roughnesspopulation, light directing and light guiding can be achieved by theoptical element of the invention. In another preferred embodiment of theinvention, the at least two roughness populations are random. Byproviding a random roughness population, light diffusion can be achievedby the optical element of the invention. Further, a random roughness hasbeen shown to reduce optical patterns that might result from an orderedroughness. In another preferred embodiment of the invention, one of theroughness populations is ordered. By providing at least one orderedpopulation, the spacer can be random, reducing unwanted transmissionpatterns and the optical element can be ordered to provide lightdirection and light guiding.

[0045] The optical element of the invention preferably comprises ageometrical spacer. A geometrical spacer, greater in height that thefunctional optical element, provides a precise air gap when used incombination with other optical components such as brightness enhancementfilms and polarizing films. A geometrical shape provides mechanical andoptical utility for both reflected and transmitted light energy. In apreferred embodiment of the invention, the geometric spacer of theinvention comprises a cylinder. A cylinder provides for specular lighttransmission and is impact resistant. Further, the end of the cylinderprovides excellent contact points when the optical element of theinvention is used in combination with other optical components.

[0046] In another preferred embodiment of the invention, the geometricalspacer comprises a sphere. The sphere provides a precision gap as wellas light diffusion as transmitted light is diffused from the curvedsurface of the geometrical spacer. In another preferred embodiment ofthe invention, the geometrical spacer comprises a cube. A cubicgeometrical spacer provides impact resistance as well as a precisionoptical spacing. In another preferred embodiment, the geometrical spacercomprises a pyramid. A pyramid provides a precision optical gap as wellas light directing. A 45 degree pyramid in air will tend to focustransmitted light from the base of the pyramid onto the top of thepyramid providing both optical spacing as well as light directing.

[0047] The geometrical spacer preferably comprises a polymer. Apolymeric geometrical spacer provides high light transmissionproperties, is inexpensive and can be easily formed into a geometricalspacer. Preferred polymers include polyolefins, polyesters, polyamides,polycarbonates, cellulosic esters, polystyrene, polyvinyl resins,polysulfonamides, polyethers, polyimides, polyvinylidene fluoride,polyurethanes, polyphenylenesulfides, polytetrafluoroethylene,polyacetals, polysulfonates, polyester ionomers, and polyolefinionomers. Copolymers and/or mixtures of these polymers can be used.Polyolefins particularly polypropylene, polyethylene, polymethylpentene,and mixtures thereof are preferred. Polyolefin copolymers, includingcopolymers of propylene and ethylene such as hexene, butene and octeneare also preferred. Polypropylenes are most preferred because they arelow in cost and have good strength and surface properties and have beenshown to be soft and scratch resistant.

[0048] In another preferred embodiment, the geometrical spacer comprisesan organic particle. Organic particles such as metals, silica, clay andthe like are useful as a spacer when high temperatures, such as thoseencountered in hot climates or industrial applications are required asthe temperature resistance is much high when compared to organic spacermaterial such as polymers.

[0049] The geometrical spacer of the invention preferably has a lighttransmission greater than 80%. A light transmission greater than 80%provides for transmitted light efficiency suitable for LC devices andreduces unwanted optical “dead spots” were geometrical spacers can bevisible in transmitted light. The geometrical spacer of the inventionpreferably has a haze greater than 60%. By providing a spacer that has ahaze of greater than 60%, the transmitted visibility of the spacer isreduced. In a preferred form, the geometrical light spacer of theinvention comprises a diffusion element such as curved lens, a complexlens or an embossed surface for the diffusion of reflected andtransmitted light.

[0050] The geometrical spacer of the invention has a frequency ofgreater than 1.0 mm. At a frequency of greater than 1.0 mm a precisionair gap is formed between the invention materials and other opticalcomponents and the frequency is on the order of 100 to 1000 times largerthan typical optical features to accomplish light direction, lightshaping or light diffusion. In another preferred embodiment, thegeometrical spacer has a frequency of between 5 and 25 mm. In thisrange, it has been shown that a precision air gap is formed andmaintained in a portable LC device. With a large geometrical spacing, aspacing that exceeds 100 mm, the air gap precision begins to degrade asthe spacing is too large to avoid the bend of optical components intothe unsupported space between the geometrical spacers.

[0051] A precision air gap between a surface light diffuser andadditional optical components is important to maintain the efficiency ofthe light diffuser. Complex lenses containing at least two roughnesspopulations wherein the at least two populations differ by at least 8micrometers have been shown to be very efficient light diffuserscompared to light diffusers whose light diffusion element is in opticalcontact with other optical components. A transparent polymeric filmhaving a top and bottom surface comprising a plurality of convex orconcave complex lenses on the surface of the transparent polymeric filmis preferred. Curved concave and convex polymer lenses have been shownto provide very efficient diffusion of light. Further, the polymerlenses of the invention are transparent, allowing a high transmission oflight allowing the brightness of LC displays to emit more light.

[0052] The concave or complex lenses on the surface of the polymer filmare preferably randomly placed. Random placement of lenses increases thediffusion efficiency of the invention materials. Further, by avoiding aconcave or convex placement of lenses that ordered, undesirable opticalinterference patterns are avoided.

[0053] In an embodiment of the invention, the concave or convex lensesare located on both sides of the transparent polymer sheet. By placingthe lenses on both sides of the transparent sheet, more efficient lightdiffusion is observed compared to the lenses of the invention on oneside. Further, the placement of the lenses on both sides of thetransparent sheet increases the focal length of the lenses furthest fromthe brightness enhancement film in a LC display device.

[0054] In one embodiment of the invention, convex lenses are present onthe top surface and convex lenses are present on the bottom surface ofthe transparent polymeric film. The placement of convex lenses on bothsides of the polymer film creates stand off from other adjacent filmsproviding the necessary air gap required for efficient diffusion by thelenses.

[0055] In another embodiment of the invention, convex lenses are presenton the top surface and concave lenses are present on the bottom surfaceof the transparent polymeric film. The placement of convex lenses on thetop side of the polymer film creates stand off from other adjacent filmsproviding the necessary air gap required for efficient diffusion by thelenses. The placement of concave lenses on the bottom side of thepolymer film creates a surface that can be in optical contact with theadjacent films and still effectively diffuse the light.

[0056] In another embodiment of the invention, concave lenses arepresent on the top surface and concave lenses are present on the bottomsurface of the transparent polymeric film. The placement of concavelenses on both sides of the polymer film creates a surface that can bein optical contact with the adjacent films on either side and stilleffectively diffuse the light.

[0057] In another embodiment of the invention, concave lenses arepresent on the top surface and convex lenses are present on the bottomsurface of the transparent polymeric film. The placement of concavelenses on the top side of the polymer film creates a surface that can bein optical contact with the adjacent films and still effectively diffusethe light. The placement of convex lenses on the bottom side of thepolymer film creates stand off from other adjacent films providing thenecessary air gap required for efficient diffusion by the lenses.

[0058] Preferably, the concave or convex lenses have an averagefrequency in any direction of between 4 and 250 complex lenses/mm. Whena film has an average of 285 complex lenses/mm creates the width of thelenses approach the wavelength of light. The lenses will impart a colorto the light passing through the lenses and change the color temperatureof the display. Less than 4 lenses/mm Creates lenses that are too largeand therefore diffuse the light less efficiently. Concave or convexlenses with an average frequency in any direction of between 22 and 66complex lenses/mm are most preferred. It has been shown that an averagefrequency of between 22 and 6 complex lenses provide efficient lightdiffusion and can be efficiently manufactured utilizing cast coatedpolymer against a randomly patterned roll.

[0059] The preferred transparent polymeric film has concave or convexlenses at an average width between 3 and 60 microns in the x and ydirection. When lenses have sizes below 1 micron the lenses impart acolor shift in the light passing through because the lenses dimensionsare on the order of the wavelength of light. When the lenses have anaverage width in the x or y direction of more than 68 microns, thelenses is too large to diffuse the light efficiently. More preferred,the concave or convex lenses at an average width between 15 and 40microns in the x and y direction. This size lenses has been shown tocreate the most efficient diffusion.

[0060] The concave or convex complex lenses comprising minor lenseswherein the diameter of the smaller lenses is preferably less than 80%,on average, the diameter of the major lens. When the diameter of theminor lens exceeds 80% of the major lens, the diffusion efficiency isdecreased because the complexity of the lenses is reduced.

[0061] The concave or convex complex lenses comprising minor lenseswherein the width in the x and y direction of the smaller lenses ispreferably between 2 and 20 microns. When minor lenses have sizes below1 micron the lenses impart a color shift in the light passing throughbecause the lenses dimensions are on the order of the wavelength oflight. When the minor lenses have sizes above 25 microns, the diffusionefficiency is decreased because the complexity of the lenses is reduced.Most preferred are the minor lenses having a width in the x and ydirection between 3 and 8 microns. This range has been shown to createthe most efficient diffusion.

[0062] Preferably, the concave or convex complex lenses comprise anolefin repeating unit. Polyolefins are low in cost and high in lighttransmission. Further, polyolefin polymers are efficiently meltextrudable and therefore can be used to create light diffusers in rollform.

[0063] In another embodiment of the invention, the concave or convexcomplex lenses comprise a carbonate repeating unit. Polycarbonates havehigh optical transmission values that allows for high light transmissionand diffusion. High light transmission provides for a brighter LC devicethan diffusion materials that have low light transmission values.

[0064] In another embodiment of the invention, the concave or convexcomplex lenses comprise an ester repeating unit. Polyesters are low incost and have good strength and surface properties. Further, polyesterpolymer is dimensionally stable at temperatures between 80 and 200degrees C. and therefore can withstand the heat generated by displaylight sources.

[0065] Preferably, the transparent polymeric film wherein the polymericfilm comprises an ester repeating unit. Polyesters are low in cost andhave good strength and surface properties. Further, polyester polymerfilm is dimensionally stable over the current range of temperaturesencountered in enclosed display devices. Polyester polymer easilyfractures allowing for die cutting of diffuser sheets for insertion intodisplay devices.

[0066] In another embodiment of the invention, the transparent polymericfilm wherein the polymeric film comprises a carbonate repeating unit.Polycarbonates have high optical transmission values compared topolyolefin polymers and therefore can improve the brightness of displaydevices.

[0067] In another embodiment of the invention, the transparent polymericfilm wherein the polymeric film comprises an olefin repeating unit.Polyolefins are low in cost and have good strength and surfaceproperties.

[0068] In another embodiment of the invention, the transparent polymericfilm wherein the polymeric film comprises a tri acetyl cellulose. Triacetyl cellulose has both high optical transmission and low opticalbirefringence allowing the diffuser of the invention to both diffuselight and reduce unwanted optical patterns.

[0069] The preferred diffuse light transmission of the diffuser materialof the invention is greater than 50%. Diffuser light transmission lessthan 45% does not let a sufficient quantity of light pass through thediffuser, thus making the diffuser inefficient. A more preferred diffuselight transmission of the lenslet film is greater than between 80 and95%. An 80% diffuse transmission allows the LC device to improve batterylife and increase screen brightness. The most preferred diffusetransmission of the transparent polymeric film is greater than 92%. Adiffuse transmission of 92% allows diffusion of the back light -sourceand maximizes the brightness of the LC device significant improving theimage quality of an LC device for outdoor use where the LC screen mustcompete with natural sunlight.

[0070] Preferably, the concave or convex lenses are semi-sphericalmeaning that the surface of each lenslet is a sector of a sphere, butnot necessarily a hemisphere. This provides excellent even diffusionover the x y plane. The semi-spherical shaped lenses scatter theincident light uniformly, ideal for a backlit display application wherethe display area need to be lit uniformly.

[0071] In another embodiment of the invention, the concave or convexlenses are aspherical meaning that width of the lenses differ in the xand y direction. This scatters light selectively over the x y plane. Forexample, a particular x y aspect ratio might produce an ellipticalscattering pattern. This would be useful in the front of a LC display,spreading the light more in the horizontal direction than the verticaldirection for increased viewing angle.

[0072] The convex or concave lenses preferably have a height/diameterratio of between 0.03 to 1.0. A height/diameter ratio of less than 0.01(very wide and shallow lenses) limits diffusivity because the lenses donot have enough curvature to efficiently spread the light. Aheight/diameter ratio of greater than 2.5 creates lenses where the anglebetween the side of the lenses and the substrate is large. This causesinternal reflection limiting the diffusion capability of the lenses.Most preferred is a height/diameter of the convex or concave lenses ofbetween 0.25 to 0.48. It has been proven that the most efficientdiffusion occurs in this range.

[0073] The number of minor lenses per major lens is preferably between 2and 60. When a major lens has one or no minor lenses, its complexity isreduced and therefore it does not diffuse as efficiently. When a majorlens has more than 70 minor lens contained on it, the width of some ofthe minor lens approaches the wavelength of light and imparts a color tothe light transmitted. Most preferred is 5 to 18 minor lenses per majorlens. This range has been shown to produce the most efficient diffusion.

[0074] The thickness of the transparent polymeric film preferably isless than 250 micrometers or more preferably between 12.5 and 50micrometers. Current design trends for LC devices are toward lighter andthinner devices. By reducing the thickness of the light diffuser to lessthan 250 micrometers, the LC devices can be made lighter and thinner.Further, by reducing the thickness of the light diffuser, brightness ofthe LC device can be improved by reducing light transmission. The morepreferred thickness of the light diffuser is between 12.5 and 50micrometers which further allows the light diffuser to be convienentlycombined with a other optical materials in an LC device such asbrightness enhancement films. Further, by reducing the thickness of thelight diffuser, the materials content of the diffuser is reduced.

[0075] Since the thermoplastic light diffuser of the invention typicallyis used in combination with other optical web materials, a lightdiffuser with an elastic modulus greater than 500 MPa is preferred. Anelastic modulus greater than 500 MPa allows for the light diffuser to belaminated with a pressure sensitive adhesive for combination with otheroptical webs materials. Further, because the light diffuser ismechanically tough, the light diffuser is better able to with stand therigors of the assembly process compared to prior art cast diffusionfilms which are delicate and difficult to assemble.

[0076]FIG. 1 illustrates a cross section of a complex lens formed on atransparent base material containing geometrical spacers suitable foruse in a liquid crystal display device. Light diffusion film containingcomplex lenses and geometrical spacers 12 comprises transparent polymerbase 20, onto which concave major lens 22 is present on the surface oftransparent polymer base 26. Polymer geometrical spacer 24 is integralto the surface, that is part of the surface, of the light diffusion film12 and above the surface of the complex lens 22. The frequency of thepolymer geometrical spacers 24 is the distance between the polymergeometrical spacer 24. Additional optical components such as a prismfilm would contact light diffuser 12 at the exposed surface of thepolymer geometrical spacer 24.

[0077]FIG. 2 illustrates a liquid crystal display device with a lightdiffuser. Visible light source 18 is illuminated and light is guidedinto light guide 2. Lamp reflector 4 is used to direct light energy intothe light guide 2, represented by an acrylic box. Reflection tape 6,reflection tape 10 and reflection film 8 are utilized to keep lightenergy from exiting the light guide 2 in an unwanted direction. Lightdiffusion film containing complex lenses and geometrical spacers 12 inthe form of a transparent polymeric film is utilized to diffuse lightenergy exiting the light guide in a direction perpendicular to the lightdiffuser. Brightness enhancement film 14 is utilized to focus the lightenergy into polarization film 16. The light diffusion film containingcomplex lenses and geometrical spacers 12 is in contact with brightnessenhancement film 14.

[0078] Polymer sheet for the transparent polymeric film comprising aplurality of convex and/or concave complex lenses on a surface thereofare generally dimensionally stable, optically clear and contain a smoothsurface. Biaxially oriented polymer sheets are preferred as they arethin and are higher in elastic modulus compared to cast coated polymersheets. Biaxially oriented sheets are conveniently manufactured byco-extrusion of the sheet, which may contain several layers, followed bybiaxial orientation. Such biaxially oriented sheets are disclosed in,for example, U.S. Pat. No. 4,764,425.

[0079] Suitable classes of thermoplastic polymers for the transparentpolymeric film include polyolefins, polyesters, polyamides,polycarbonates, cellulosic esters, polystyrene, polyvinyl resins,polysulfonamides, polyethers, polyimides, polyvinylidene fluoride,polyurethanes, polyphenylenesulfides, polytetrafluoroethylene,polyacetals, polysulfonates, polyester ionomers, and polyolefinionomers. Copolymers and/or mixtures of these polymers can be used.

[0080] Polyolefins particularly polypropylene, polyethylene,polymethylpentene, and mixtures thereof are preferred. Polyolefincopolymers, including copolymers of propylene and ethylene such ashexene, butene and octene are also preferred. Polypropylenes are mostpreferred because they are low in cost and have good strength andsurface properties.

[0081] Preferred polyesters for the transparent polymeric film of theinvention include those produced from aromatic, aliphatic orcycloaliphatic dicarboxylic acids of 4-20 carbon atoms and aliphatic oralicyclic glycols having from 2-24 carbon atoms. Examples of suitabledicarboxylic acids include terephthalic, isophthalic, phthalic,naphthalene dicarboxylic acid, succinic, glutaric, adipic, azelaic,sebacic, fumaric, maleic, itaconic, 1,4-cyclohexanedicarboxylic,sodiosulfoisophthalic and mixtures thereof Examples of suitable glycolsinclude ethylene glycol, propylene glycol, butanediol, pentanediol,hexanediol, 1,4-cyclohexanedimethanol, diethylene glycol, otherpolyethylene glycols and mixtures thereof. Such polyesters are wellknown in the art and may be produced by well known techniques, e.g.,those described in U.S. Pat. No. 2,465,319 and U.S. Pat. No. 2,901,466.Preferred continuous matrix polyesters are those having repeat unitsfrom terephthalic acid or naphthalene dicarboxylic acid and at least oneglycol selected from ethylene glycol, 1,4-butanediol and1,4-cyclohexanedimethanol. Poly(ethylene terephthalate), which may bemodified by small amounts of other monomers, is especially preferred.Other suitable polyesters include liquid crystal copolyesters formed bythe inclusion of suitable amount of a co-acid component such as stilbenedicarboxylic acid. Examples of such liquid crystal copolyesters arethose disclosed in U.S. Pat. Nos. 4,420,607, 4,459,402 and 4,468,510.

[0082] Useful polyamides for the transparent polymeric film includenylon 6, nylon 66, and mixtures thereof. Copolymers of polyamides arealso suitable continuous phase polymers. An example of a usefulpolycarbonate is bisphenol-A polycarbonate. Cellulosic esters suitablefor use as the continuous phase polymer of the composite sheets includecellulose nitrate, cellulose triacetate, cellulose diacetate, celluloseacetate propionate, cellulose acetate butyrate, and mixtures orcopolymers thereof. Useful polyvinyl resins include polyvinyl chloride,poly(vinyl acetal), and mixtures thereof. Copolymers of vinyl resins canalso be utilized.

[0083] The complex lenses of the invention preferably comprise polymers.Polymers are preferred as they are generally lower in cost compared toprior art glass lenses, have excellent optical properties and can beefficiently formed into lenses utilizing known processes such as meltextrusion, vacuum forming and injection molding. Preferred polymers forthe formation of the complex lenses include polyolefins, polyesters,polyamides, polycarbonates, cellulosic esters, polystyrene, polyvinylresins, polysulfonamides, polyethers, polyimides, polyvinylidenefluoride, polyurethanes, polyphenylenesulfides, polytetrafluoroethylene,polyacetals, polysulfonates, polyester ionomers, and polyolefinionomers. Copolymers and/or mixtures of these polymers to improvemechanical or optical properties can be used. Preferred polyamides forthe transparent complex lenses include nylon 6, nylon 66, and mixturesthereof. Copolymers of polyamides are also suitable continuous phasepolymers. An example of a useful polycarbonate is bisphenol-Apolycarbonate. Cellulosic esters suitable for use as the continuousphase polymer of the complex lenses include cellulose nitrate, cellulosetriacetate, cellulose diacetate, cellulose acetate propionate, celluloseacetate butyrate, and mixtures or copolymers thereof. Preferredpolyvinyl resins include polyvinyl chloride, poly(vinyl acetal), andmixtures thereof. Copolymers of vinyl resins can also be utilized.Preferred polyesters for the complex lens of the invention include thoseproduced from aromatic, aliphatic or cycloaliphatic dicarboxylic acidsof 4-20 carbon atoms and aliphatic or alicyclic glycols having from 2-24carbon atoms. Examples of suitable dicarboxylic acids includeterephthalic, isophthalic, phthalic, naphthalene dicarboxylic acid,succinic, glutaric, adipic, azelaic, sebacic, fumaric, maleic, itaconic,1,4-cyclohexanedicarboxylic, sodiosulfoisophthalic and mixtures thereofExamples of suitable glycols include ethylene glycol, propylene glycol,butanediol, pentanediol, hexanediol, 1,4-cyclohexanedimethanol,diethylene glycol, other polyethylene glycols and mixtures thereof.

[0084] Addenda is preferably added to a polyester skin layer to changethe color of the imaging element. An addenda of this invention thatcould be added is an optical brightener. An optical brightener issubstantially colorless, fluorescent, organic compound that absorbsultraviolet light and emits it as visible blue light. Examples includebut are not limited to derivatives of4,4′-diaminostilbene-2,2′-disulfonic acid, coumarin derivatives such as4-methyl-7-diethylaminocoumarin, 1-4-Bis (O-Cyanostyryl) Benzol and2-Amino-4-Methyl Phenol. An unexpected desirable feature of thisefficient use of optical brightener. Because the ultraviolet source fora transmission display material is on the opposite side of the image,the ultraviolet light intensity is not reduced by ultraviolet filterscommon to imaging layers. The result is less optical brightener isrequired to achieve the desired background color.

[0085] The diffuser sheets may be coated or treated before or afterthermoplastic lenslet casting with any number of coatings which may beused to improve the properties of the sheets including printability, toprovide a vapor barrier, to make them heat sealable, or to improveadhesion. Examples of this would be acrylic coatings for printability,coating polyvinylidene chloride for heat seal properties. Furtherexamples include flame, plasma or corona discharge treatment to improveprintability or adhesion.

[0086] The diffuser sheets of the present invention may be used incombination with one or more layers selected from an opticalcompensation film, a polarizing film and a substrate constitution aliquid crystal layer. The diffusion film of the present invention ispreferably used by a combination of diffusion film/polarizingfilm/optical compensation film in that order. In the case of using theabove films in combination in a liquid crystal display device, the filmscould be bonded with each other e.g. through a tacky adhesive forminimizing the reflection loss, etc. The tacky adhesive is preferablythose having a refractive index close to that of the oriented film tosuppress the interfacial reflection loss of light.

[0087] The lenslet diffuser film may also be used in conjunction withanother light diffuser, for example a bulk diffuser, a lenticular layer,a beaded layer, a surface diffuser, a holographic diffuser, amicro-structured diffuser, another lens array, or various combinationsthereof The lenslet diffuser film disperses, or diffuses, the light,thus destroying any diffraction pattern that may arise from the additionof an ordered periodic lens array. The lenslet diffuser film may bepositioned before or after any diffuser or lens array.

[0088] The diffusion sheet of the present invention may be used incombination with a film or sheet made of a transparent polymer. Examplesof such polymer are polyesters such as polycarbonate, polyethyleneterephthalate, polybutylene terephthalate and polyethylene naphthalate,acrylic polymers such as polymethyl methacrylate, and polyethylene,polypropylene, polystyrene, polyvinyl chloride, polyether sulfone,polysulfone, polyacrylate and triacetyl cellulose. The bulk diffuserlayer may be mounted to a glass sheet for support.

[0089] The transparent polymeric film of the invention can also include,in another aspect, one or more optical coatings to improve opticaltransmission through one or more lenslet channels. It is often desirableto coat a diffuser with a layer of an anti-reflective (AR) coating inorder to raise the efficiency of the diffuser.

[0090] The diffuser sheet of the present invention may be incorporatedwith e.g. an additive or a lubricant such as silica for improving thesurface-slipperiness of the film within a range not to deteriorate theoptical characteristics to vary the light-scattering property with anincident angle. Examples of such additive are organic solvents such asxylene, alcohols or ketones, fine particles of an acrylic resin,silicone resin or A metal oxide or a filler.

[0091] The lenslet diffuser film of the present invention usually hasoptical anisotropy. The web material and the casted thermoplastic resinare generally optically anisotropic materials exhibiting opticalanisotropy having an optic axis in the drawing direction. The opticalanisotropy is expressed by the product of the film thickness d and thebirefringence Δn which is a difference between the refractive index inthe slow optic axis direction and the refractive index in the fast opticaxis direction in the plane of the film, i.e. Δn * d (retardation). Theorientation direction coincides with the drawing axis in the film of thepresent invention. The drawing axis is the direction of the slow opticaxis in the case of a thermoplastic polymer having a positive intrinsicbirefringence and is the direction of the fast optic axis for athermoplastic polymer having a negative intrinsic birefringence. Thereis no definite requirement for the necessary level of the value of Δn. *d since the level depends upon the application of the film.

[0092] In the manufacturing process for this invention, preferred lenspolymers are melt extruded from a slit die. In general, a T die or acoat hanger die are preferably used. The process involves extruding thepolymer or polymer blend through a slit die and rapidly quenching theextruded web upon a chilled casting drum with the preferred lensgeometry so that the lens polymer component of the transparent sheet arequenched below their glass solidification temperature and retain theshape of the diffusion lens.

[0093] A method of fabricating a diffusion film containing twopopulation of roughness was developed. The preferred approach comprisesthe steps of providing a positive master chill roll having a pluralityof complex lenses. The diffusion film is replicated from the masterchill roller by casting a molten polymeric material to the face of thechill roll and transferring the polymeric material with two roughnesspopulations with complex lens structures onto a transparent polymericfilm. In another embodiment of the invention, a geometrical spacer isformed by forming a roller containing the complex lens geometry and theymachining in a geometrical shape such as a cylinder to form a rollcontaining both the random complex lens geometry and number of cylindershaped geometrical spacers.

[0094] A chill roller is manufactured by a process including the stepsof electroplating a layer of copper onto the surface of a roller, andthen abrasively blasting the surface of the copper layer with beads,such as glass or silicon dioxide, to create a surface texture withhemispherical features. The resulting blasted surface is bright nickelelectroplated or chromed to a depth that results in a surface texturewith the features either concave into the roll or convex out of theroll. Because of the release characteristics of the chill roll surface,the resin will not adhere to the surface of the roller.

[0095] The bead blasting operation is carried out using an automateddirect pressure system in which the nozzle feed rate, nozzle distancefrom the roller surface, the roller rotation rate during the blastingoperation and the velocity of the particles are accurately controlled tocreate the desired lenslet structure.

[0096] The number of features in the chill roll per area is determinedby the bead size and the pattern depth. Larger bead diameters and deeperpatterns result in fewer numbers of features in a given area. Thereforethe number of features is inherently determined by the bead size and thepattern depth.

[0097] The optical element and the geometrical spacer of the inventionmay also be manufactured by vacuum forming around a pattern, injectionmolding the lenses and embossing lenses in a polymer web. While thesemanufacturing techniques do yield acceptable lenses capable ofefficiently diffusing light, melt cast coating polymer onto a patternedroll and subsequent transfer onto a transparent polymer web allows forthe lenses of the invention to be formed into rolls thereby lowering themanufacturing cost for the diffusion lenses. Further, cast coatingpolymer has been shown to more efficiently replicate the desired complexlens geometry compared to embossing and vacuum forming.

[0098] In another embodiment, polymer beads of differing mean diametersare preferably coated in a matrix on a polymer transparent web. The meandiameter difference between the two populations of beads provides thedesired geometrical spacing between the optical element of the inventionand other optical components. For example, two populations ofpolystyrene polymer beads coated in a PVA matrix, one at 10 micrometersand the other at 50 micrometers, provides a 40 micrometer geometricalspacer on the surface of the transparent polymer web.

[0099] The optical spacers of the invention are preferably located onthe surface of a voided polymer light diffuser. By providing an opticalspacer, the diffusion cone of the voided polymer can be preciselycontrolled. The invention provides a film that scatters the incidentlight uniformly. The oriented film of the present invention can beproduced by using a conventional film-manufacturing facility in highproductivity. The invention utilizes a voided thermal plastic layercontaining microvoids. Microvoids of air in a polymer matrix arepreferred and have been shown to be a very efficient diffuser of lightcompared to prior art diffuser materials which rely on surface roughnesson a polymer sheet to create light diffusion for LCD devices. Themicrovoided layers containing air have a large index of refractiondifference between the air contained in the voids (n=1) and the polymermatrix (n=1.2 to 1.8). This large index of refraction differenceprovides excellent diffusion and high light transmission which allowsthe LCD image to be brighter and/or the power requirements for the lightto be reduces thus extending the life of a battery. The preferreddiffuse light transmission of the diffuser material of the invention aregreater than 65%. Diffuser light transmission less than 60% does not leta sufficient quantity of light pass through the diffuser, thus makingthe diffuser inefficient. A more preferred diffuse light transmission ofthe microvoided thermoplastic voided layer is greater than 80%. An 80%diffuse transmission allows the LC device to improve battery life andincrease screen brightness. The most preferred diffuse transmission ofthe voided thermoplastic layer is greater than 87%. A diffusetransmission of 87% allows diffusion of the back light source andmaximizes the brightness of the LC device significant improving theimage quality of an LC device for outdoor use where the LC screen mustcompete with natural sunlight.

[0100] Since the microvoids of the invention are substantially air, theindex of refraction of the air containing voids is 1. An index ofrefraction difference between the air void and the thermoplastic matrixis preferably greater than 0.2. An index of refraction differencegreater than 0.2 has been shown to provide excellent diffusion of LCDback light sources and a index of refraction difference of greater than0.2 allows for bulk diffusion in a thin film which allows LCDmanufacturers to reduce the thickness of the LC screen. Thethermoplastic diffusion layer preferably contains at least 4 index ofrefraction changes greater than 0.2 in the vertical direction. Greaterthan 4 index of refraction changes have been shown to provide enoughdiffusion for most LC devices. 30 or more index of refractiondifferences in the vertical direction, while providing excellentdiffusion, significantly reduces the amount of transmitted light,significantly reducing the brightness of the LC device.

[0101] Since the thermoplastic light diffuser of the invention typicallyis used in combination with other optical web materials, a lightdiffuser with an elastic modulus greater than 500 MPa is preferred. Anelastic modulus greater than 500 MPa allows for the light diffuser to belaminated with a pressure sensitive adhesive for combination with otheroptical webs materials. Further, because the light diffuser ismechanically tough, the light diffuser is better able to with stand therigors of the assembly process compared to prior art cast diffusionfilms which are delicate and difficult to assemble. A light diffuserwith an impact resistance greater than 0.6 GPa is preferred. An impactresistance greater than 0.6 GPa allows the light diffuser to resistscratching and mechanical deformation that can cause unwanted unevendiffusion of the light causing “hot” spots in an LC device.

[0102] The invention may be used in conjunction with any liquid crystaldisplay devices, typical arrangements of which are described in thefollowing. Liquid crystals (LC) are widely used for electronic displays.In these display systems, a LC layer is situated between a polarizerlayer and an analyzer layer and has a director exhibiting an azimuthaltwist through the layer with respect to the normal axis. The analyzer isoriented such that its absorbing axis is perpendicular to that of thepolarizer. Incident light polarized by the polarizer passes through aliquid crystal cell is affected by the molecular orientation in theliquid crystal, which can be altered by the application of a voltageacross the cell. By employing this principle, the transmission of lightfrom an external source, including ambient light, can be controlled. Theenergy required to achieve this control is generally much less than thatrequired for the luminescent materials used in other display types suchas cathode ray tubes. Accordingly, LC technology is used for a number ofapplications, including but not limited to digital watches, calculators,portable computers, electronic games for which light weight, low powerconsumption and long operating life are important features.

[0103] Active-matrix liquid crystal displays (LCDs) use thin filmtransistors (TFTs) as a switching device for driving each liquid crystalpixel. These LCDs can display higher-definition images without crosstalk because the individual liquid crystal pixels can be selectivelydriven. Optical mode interference (OMI) displays are liquid crystaldisplays, which are “normally white,” that is, light is transmittedthrough the display layers in the off state. Operational mode of LCDusing the twisted nematic liquid crystal is roughly divided into abirefringence mode and an optical rotatory mode. “Film-compensatedsuper-twisted nematic” (FSTN) LCDs are normally black, that is, lighttransmission is inhibited in the off state when no voltage is applied.OMI displays reportedly have faster response times and a broaderoperational temperature range.

[0104] Ordinary light from an incandescent bulb or from the sun israndomly polarized, that is, it includes waves that are oriented in allpossible directions. A polarizer is a dichroic material that functionsto convert a randomly polarized (“unpolarized”) beam of light into apolarized one by selective removal of one of the two perpendicularplane-polarized components from the incident light beam. Linearpolarizers are a key component of liquid-crystal display (LCD) devices.

[0105] There are several types of high dichroic ratio polarizerspossessing sufficient optical performance for use in LCD devices. Thesepolarizers are made of thin sheets of materials which transmit onepolarization component and absorb the other mutually orthogonalcomponent (this effect is known as dichroism). The most commonly usedplastic sheet polarizers are composed of a thin, uniaxially-stretchedpolyvinyl alcohol (PVA) film which aligns the PVA polymer chains in amore-or-less parallel fashion. The aligned PVA is then doped with iodinemolecules or a combination of colored dichroic dyes (see, for example,EP 0 182 632 A2, Sumitomo Chemical Company, Limited) which adsorb to andbecome uniaxially oriented by the PVA to produce a highly anisotropicmatrix with a neutral gray coloration. To mechanically support thefragile PVA film it is then laminated on both sides with stiff layers oftriacetyl cellulose (TAC), or similar support.

[0106] Contrast, color reproduction, and stable gray scale intensitiesare important quality attributes for electronic displays, which employliquid crystal technology. The primary factor limiting the contrast of aliquid crystal display is the propensity for light to “leak” throughliquid crystal elements or cell, which are in the dark or “black” pixelstate. Furthermore, the leakage and hence contrast of a liquid crystaldisplay are also dependent on the angle from which the display screen isviewed. Typically the optimum contrast is observed only within a narrowviewing angle centered about the normal incidence to the display andfalls off rapidly as the viewing angle is increased. In color displays,the leakage problem not only degrades the contrast but also causes coloror hue shifts with an associated degradation of color reproduction. Inaddition to black-state light leakage, the narrow viewing angle problemin typical twisted nematic liquid crystal displays is exacerbated by ashift in the brightness-voltage curve as a function of viewing anglebecause of the optical anisotropy of the liquid crystal material.

[0107] The transparent polymeric film of the present invention can evenout the luminance when the film is used as a light-scattering film in abacklight system. Back-lit LCD display screens, such as are utilized inportable computers, may have a relatively localized light source (ex.fluorescent light) or an array of relatively localized light sourcesdisposed relatively close to the LCD screen, so that individual “hotspots” corresponding to the light sources may be detectable. Thediffuser film serves to even out the illumination across the display.The liquid crystal display device includes display devices having acombination of a driving method selected from e.g. active matrix drivingand simple matrix drive and a liquid crystal mode selected from e.g.twist nematic, supertwist nematic, ferroelectric liquid crystal andantiferroelectric liquid crystal mode, however, the invention is notrestricted by the above combinations. In a liquid crystal displaydevice, the oriented film of the present invention is necessary to bepositioned in front of the backlight. The lenslet diffuser film of thepresent invention can even the lightness of a liquid crystal displaydevice across the display because the film has excellentlight-scattering properties to expand the light to give excellentvisibility in all directions. Although the above effect can be achievedeven by the single use of such lenslet diffuser film, plural number offilms may be used in combination. The homogenizing lenslet diffuser filmmay be placed in front of the LCD material in a transmission mode todisburse the light and make it much more homogenous. The presentinvention has a significant use as a light source destructuring device.In many applications, it is desirable to eliminate from the output ofthe light source itself the structure of the filament which can beproblematic in certain applications because light distributed across thesample will vary and this is undesirable. Also, variances in theorientation of a light source filament or arc after a light source isreplaced can generate erroneous and misleading readings. A homogenizinglenslet diffuser film of the present invention placed between the lightsource and the detector can eliminate from the output of the lightsource any trace of the filament structure and therefore causes ahomogenized output which is identical from light source to light source.

[0108] The lenslet diffuser films may be used to control lighting forstages by providing pleasing homogenized light that is directed wheredesired. In stage and television productions, a wide variety of stagelights must be used to achieve all the different effects necessary forproper lighting. This requires that many different lamps be used whichis inconvenient and expensive. The films of the present invention placedover a lamp can give almost unlimited flexibility dispersing light whereit is needed. As a consequence, almost any object, moving or not, and ofany shape, can be correctly illuminated.

[0109] The reflection film formed by applying a reflection layercomposed of a metallic film, etc., to the lenslet diffuser film of thepresent invention can be used e.g. as a retroreflective member for atraffic sign. It can be used in a state applied to a car, a bicycle,person, etc.

[0110] The lenslet diffuser films of the present invention may also beused in the area of law enforcement and security systems to homogenizethe output from laser diodes (LDs) or light emitting diodes (LEDs) overthe entire secured area to provide higher contrasts to infrared (IR)detectors. The films of the present invention may also be used to removestructure from devices using LED or LD sources such as in bank notereaders or skin treatment devices. This leads to greater accuracy.

[0111] Fiber-optic light assemblies mounted on a surgeon's headpiece cancast distracting intensity variations on the surgical field if one ofthe fiber-optic elements breaks during surgery. A lenslet diffuser filmof the present invention placed at the ends of the fiber bundlehomogenizes light coming from the remaining fibers and eliminates anytrace of the broken fiber from the light cast on the patient. A standardground glass diffuser would not be as effective in this use due tosignificant back-scatter causing loss of throughput.

[0112] The lenslet diffuser films of the present invention can also beused to homogeneously illuminate a sample under a microscope bydestructuring the filament or arc of the source, yielding ahomogeneously illuminated field of view. The films may also be used tohomogenize the various modes that propagate through a fiber, forexample, the light output from a helical-mode fiber.

[0113] The lenslet diffuser films of the present invention also havesignificant architectural uses such as providing appropriate light forwork and living spaces. In typical commercial applications, inexpensivetransparent polymeric diffuser films are used to help diffuse light overthe room. A homogenizer of the present invention which replaces one ofthese conventional diffusers provides a more uniform light output sothat light is diffused to all angles across the room evenly and with nohot spots.

[0114] The lenslet diffuser films of the present invention may also beused to diffuse light illuminating artwork. The transparent polymericfilm diffuser provides a suitable appropriately sized and directedaperture for depicting the artwork in a most desirable fashion.

[0115] Further, the lenslet diffuser film of the present invention canbe used widely as a part for an optical equipment such as a displayingdevice. For example, it can be used as a light-reflection platelaminated with a reflection film such as a metal film in a reflectiveliquid crystal display device or a front scattering film directing thefilm to the front-side (observer's side) in the case of placing themetallic film to the back side of the device (opposite to the observer),in addition to the aforementioned light-scattering plate of a backlightsystem of a liquid crystal display device. The lenslet diffuser film ofthe present invention can be used as an electrode by laminating atransparent conductive layer composed of indium oxide represented by ITOfilm. If the material is to be used to form a reflective screen, e.g.front projection screen, a light-reflective layer is applied to thetransparent polymeric film diffuser.

[0116] Another application for the transparent polymeric diffuser filmis a rear projection screen, where it is generally desired to projectthe image from a light source onto a screen over a large area. Theviewing angle for a television is typically smaller in the verticaldirection than in the horizontal direction. The diffuser acts to spreadthe light to increase viewing angle.

[0117] Diffusion film samples were measured with the Hitachi U4001UV/Vis/NIR spectrophotometer equipped with an integrating sphere. Thetotal transmittance spectra were measured by placing the samples at thebeam port with the front surface with complex lenses towards theintegrating sphere. A calibrated 99% diffusely reflecting standard(NIST-traceable) was placed at the normal sample port. The diffusetransmittance spectra were measured in like manner, but with the 99%tile removed. The diffuse reflectance spectra were measured by placingthe samples at the sample port with the coated side towards theintegrating sphere. In order to exclude reflection from a samplebacking, nothing was placed behind the sample. All spectra were acquiredbetween 350 and 800 nm. As the diffuse reflectance results are quotedwith respect to the 99% tile, the values are not absolute, but wouldneed to be corrected by the calibration report of the 99% tile.

[0118] Percentage total transmitted light refers to percent of lightthat is transmitted though the sample at all angles. Diffusetransmittance is defined as the percent of light passing though thesample excluding a 2.5 degree angle from the incident light angle. Thediffuse light transmission is the percent of light that is passedthrough the sample by diffuse transmittance. Diffuse reflectance isdefined as the percent of light reflected by the sample. The percentagesquoted in the examples were measured at 500 nm. These values may not addup to 100% due to absorbencies of the sample or slight variations in thesample measured.

[0119] Embodiments of the invention may provide not only improved lightdiffusion and transmission but also a diffusion film of reducedthickness, and that has reduced light scattering tendencies.

[0120] The entire contents of the patents and other publicationsreferred to in this specification are incorporated herein by reference.

EXAMPLES

[0121] In this example, complex light diffusion lenses of the inventionwere created by extrusion casting an extrusion grade polyolefin polymeragainst a pattered chill roll containing the complex lens geometry. Thepatterned chill roll also contained cylindrical spacers that extendedbeyond the surface of the complex light diffusion lenses. The patternedpolyolefin polymer, in the form the complex lens and cylindrical spacerswas then transferred to a polyester web material thereby forming a lightdiffuser with complex surface lenses and integral cylindrical spacers.This example will show that the cylindrical spacers added to the surfaceof the complex light diffusion lenses improve the light diffusion cone.Further, it will be obvious that the light diffuser containing thecylindrical spacers will be low in cost and have mechanical propertiesthat allows for insertion into LC devices.

[0122] A patterned chill roll containing complex light diffusion lensesand cylindrical spacers was manufactured by a process including thesteps of electroplating a layer of cooper onto the surface of a roller,and then abrasively blasting the surface of the copper layer with glassbeads to create a surface texture with hemispherical features. Theresulting blasted surface was bright nickel electroplated to a depththat results in a surface texture with the features concave into theroll. The bead blasting operation was carried out using an automateddirect pressure system in which the nozzle feed rate, nozzle distancefrom the roller surface, the roller rotation rate during the blastingoperation and the velocity of the particles are accurately controlled tocreate the desired complex lens structure. The number of features in thechill roll per area is determined by the bead size and the patterndepth. Larger bead diameters and deeper patterns result in fewer numbersof features in a given area.

[0123] The complex lens patterned roll was manufactured by starting witha steel roll blank and grit blasted with size 14 grit at a pressure of447 MPa. The roll was then chrome platted. The resulting complex lenseson the surface of the roll were convex. The single lens patterned roll(control) was manufactured by starting with a copper roll blank and gritblasted with size 14 spherical grit at a pressure of 310 MPa. Theresulting single lenses on the surface of the roll were concave. Afterapplication of the complex lenses geometry to the surface of thepatterned roll, cylindrical spacers at depths of 50, 100 and 150micrometers were machined into the roll utilizing precision drillingtooling. The cylindrical spacers were spaced in an ordered pattern witha 10 mm spacing between each cylindrical spacer.

[0124] The patterned chill roll containing the complex lenses and thecylindrical spacers were utilized to create light diffusion sheets byextrusion coating a polyolefin polymer from a coat hanger slot diecomprising substantially 96.5% LDPE (Eastman Chemical grade D4002P), 3%Zinc Oxide and 0.5% of calcium stearate onto a 100 micrometertransparent oriented web polyester web with a % light transmission of97.2 %. The polyolefin cast coating coverage was 25.88 g/m².

[0125] The invention materials containing complex lenses had randomlydistributed lenses comprising a major lens with an average diameter of27.1 micrometers and minor lenses on the surface of the major lenseswith an average diameter of 6.7 micrometers. The average minor to majorlens ratio was 17.2 to 1. The control diffusion sheet comprisingrandomly distributed single lenses with an average diameter of 25.4micrometers. The chill roller contained separate areas for each of thethree cylindrical spacers. The structure of the cast coated diffusionsheets is as follows,

[0126] Formed polyolefin lenses with cylindrical spacers

[0127] Transparent polyester base

[0128] The light diffusion sheets containing formed polymer lenses andcylindrical spacers from above were measured for % light transmission, %diffuse light transmission, % specular light transmission and % diffusereflectance. Diffusion film samples were measured with the Hitachi U4001UV/Vis/NIR spectrophotometer equipped with an integrating sphere. Thetotal transmittance spectra were measured by placing the samples at thebeam port with the front surface with complex lenses towards theintegrating sphere. A calibrated 99% diffusely reflecting standard(NIST-traceable) was placed at the normal sample port. The diffusetransmittance spectra were measured in like manner, but with the 99%tile removed. The diffuse reflectance spectra were measured by placingthe samples at the sample port with the coated side towards theintegrating sphere. In order to exclude reflection from a samplebacking, nothing was placed behind the sample. All spectra were acquiredbetween 350 and 800 nm. As the diffuse reflectance results are quotedwith respect to the 99% tile, the values are not absolute, but wouldneed to be corrected by the calibration report of the 99% tile.

[0129] Percentage total transmitted light refers to percent of lightthat is transmitted though the sample at all angles. Diffusetransmittance is defined as the percent of light passing though thesample excluding a 2.5 degree angle from the incident light angle. Thediffuse light transmission is the percent of light that is passedthrough the sample by diffuse transmittance. Diffuse reflectance isdefined as the percent of light reflected by the sample. The percentagesquoted in the examples were measured at 500 nm. These values may not addup to 100% due to absorbencies of the sample or slight variations in thesample measured. The measured values for the invention are listed inTable 1 below. TABLE 1 Invention Total transmission 92.4 measured at 500nm Diffuse transmission 89.1 measured at 500 nm Spectral transmission3.5 measured at 500 nm Diffuse reflectance 2.6 measured at 500 nm

[0130] The invention materials were then assembled into an opticalsystem by applying the complex lenses and the four cylindrical spacers(0, 50, 100 and 150 micrometers) to a 100 micrometer thick sheet ofpolyester that had a % light transmission of 93%. The inventionmaterials were imaged through a detector, located behind the transparentpolyester sheet using a 633 nanometer 0.02 W/cm² laser with a 3.0 mmbeam diameter. The cone diameter for the four cylindrical spacers wascalculated. The cone diameter was determined when the intensity droppedoff to 10% of the peak intensity (normal to the detector). The conediameter (measured in mm) for the cylindrical spacers is listed in Table2 below, TABLE 2 Height of the Geometrical Spacer (micrometers) ConeDiameter (mm) 0 3.1 50 3.7 100 4.3 150 4.8

[0131] As the data in Table 1 above clearly indicates, complex polymerlenses formed on the surface of a transparent polymer provide excellentlight diffusion and % transmission allowing for brighter liquid crystaldisplay devices. The diffuse light transmission of 89.1% for theinvention material is significantly better than typical prior artmaterials which generally have a % light transmission of 70%. Thecomplex lens of the invention provides significantly more curved surfacearea for transmitted light diffusion compared to the prior art materialswhich typically comprise polymer beads coated in a matrix. Diffuse lighttransmission is important factor in the quality of a LC device in thatthe diffusion sheet must mask the pattern of the light guide common toLC devices. The total light transmission of the invention of 92.4% isalso significantly improved over the prior art materials. By providing alens that reduces internal scattering and reflection back toward thesource, the invention materials allow for 92.4% of the light energy topass through the diffuser resulting in a brighter liquid crystaldisplay.

[0132] The data in Table 2 clearly demonstrate the utility of ageometrical spacer. As the geometrical spacing increases, the diameterof the light diffusion cone increased providing a tunable lightdiffuser. The precision spacing provided by the invention materialallows for a fixed air gap when the invention materials are utilizedwith other optical components. When the height of the geometrical spacerwas zero, the 3 mm light source was focused (3.1 mm) exiting the lightdiffuser. This light diffusion cone has commercial value in displaydevices that require a small but intense diffusion cone, devices such asa LC cell phone or a LC automobile gauge. At a spacing of 150micrometers, the cone diameter increased to 4.8 mm providing a lessintense but wider diffusion cone providing improvements to LCtelevisions or projection televisions were viewing angle and illuminanceover a wide angle is valued.

[0133] Further, because the invention materials were constructed on anoriented polyester base, the materials have a higher elastic moduluscompared to cast diffuser sheets. The geometrical spacer were integralto the surface of the light diffuser of the invention and therefore werelow in cost and structurally attached to the surface of the lightdiffuser reducing the probability of detachment during the lifetime ofthe light diffuser. The geometrical spacers also provided separationbetween the diffuser sheet and the transparent polyester sheet, reducingthe opportunity for scratching that can occur in portable devices thatencounter significant vibration. Because the geometrical spacers of theinvention comprise low density polyethylene, the geometrical spacerswere soft and less prone to scratching compared to acrylic or PMMApolymers.

[0134] The geometrical spacer also provides impact resistance as theimpact energy that is generated when a portable display device such as acell phone is accidentally dropped, the air gap and the geometricalspacer have been shown to absorb the impact energy just as a mechanicalspring would absorb energy, and reduce the impact forces on an LC matrixwhich has been typically constructed of glass. The precision air gapalso creates thermal insulation between the high intensity light sourcestypical of LC devices and the delicate LC materials, which are heatsensitive. The insulation layer created by the geometrical spacer canprovide longer LC lifetime and reduce the imaging variability caused byheating from the high intensity light sources.

[0135] While this example was primarily directed toward the use ofthermoplastic light diffusion materials containing geometrical spacersfor LC devices, the materials of the invention have value in otherdiffusion applications such as back light display, imaging elementscontaining a diffusion layer, a diffuser for specular home lighting andprivacy screens, organic light emitting displays, image capturediffusion lenses and greenhouse light diffusion. The geometrical spacersalso have value when used with other optical elements such as lightdirectors, prism sheet, light guides and color filters.

[0136] Parts List

[0137]2. Light guide

[0138]4. Lamp Reflector

[0139]6. Reflection tape

[0140]8. Reflection film

[0141]10. Reflection tape

[0142]12. Light diffusion film containing complex lenses and geometricalspacers

[0143]14. Brightness enhancement film

[0144]16. Polarization film

[0145]18. Visible light source

[0146]20. Transparent polymer base

[0147]22. Complex polymer light diffusion lens

[0148]24. Polymer geometrical spacer

[0149]26. Surface of transparent polymer base

What is claimed is:
 1. An optical element containing a rough surfacehaving a roughness average equal to at least 5 micrometers wherein therough surface contains at least two roughness populations in which theroughness average of the at least two populations varies by at least 8micrometers.
 2. The optical element of claim 1 wherein said opticalelement has a top and bottom surface containing said rough surface. 3.The optical element of claim 1 wherein the roughness average of the saidat least two populations varies by at least 75 micrometers.
 4. Theoptical element of claim 1 wherein the roughness average of the said atleast two populations varies by at least 250 micrometers.
 5. The opticalelement of claim 1 wherein said rough surface has a roughness averageequal to a least 15 micrometers.
 6. The optical element of claim 1wherein said at least two roughness populations are ordered.
 7. Theoptical element of claim 1 wherein said at least two roughnesspopulations are random.
 8. The optical element of claim 1 wherein saidat least two roughness populations comprise one population that isordered.
 9. The optical element of claim 1 wherein said at least tworoughness populations further comprise an integral geometric spacer. 10.The optical element of claim 9 wherein said integral geometric spacercomprises a cylinder.
 11. The optical element of claim 9 wherein saidintegral geometric spacer comprises a sphere.
 12. The optical element ofclaim 9 wherein said integral geometric spacer comprises a cube.
 13. Theoptical element of claim 9 wherein said integral geometric spacercomprises a pyramid.
 14. The optical element of claim 9 wherein saidintegral geometric spacer comprises a polymer.
 15. The optical elementof claim 9 wherein said integral geometric spacer comprises an inorganicparticle.
 16. The optical element of claim 9 wherein said integralgeometric spacer has a light transmission greater than 80%.
 17. Theoptical element of claim 9 wherein said integral geometric spacer has ahaze of greater than 60%.
 18. The optical element of claim 9 whereinsaid integral geometric spacer has a frequency of greater than 1.0 mm.19. The optical element of claim 9 wherein said integral geometricspacer has a frequency between 5 and 25 mm.
 20. The optical element ofclaim 1 further comprising a light diffuser.
 21. The light diffuser ofclaim 20 comprising a transparent polymeric film having a top and bottomsurface comprising a plurality of complex lenses on at least one surfacethereof.
 22. The light diffuser of claim 21 comprising a transparentpolymeric film wherein the complex lenses are randomly distributed onthe surface.
 23. The light diffuser of claim 21 wherein said complexlenses are concave lenses.
 24. The light diffuser of claim 21 whereinthe complex lenses have an average width in the x and y direction in theplane of the film of 3 to 60 microns.
 25. The light diffuser of claim 21wherein the complex lenses comprise minor lenses wherein the diameter ofthe smaller lenses is on average less than 80% of the diameter of themajor lens they are associated with.
 26. The light diffuser of claim 21wherein the diffuse light transmission is 80 to 95%.
 27. The lightdiffuser of claim 21 wherein the complex lenses are semi-spherical. 28.The light diffuser of claim 21 wherein the complex lenses areaspherical.
 29. The light diffuser of claim 21 wherein the complexlenses have a height/diameter ratio of 0.03 to 1.0.
 30. The lightdiffuser of claim 21 comprising a thermoplastic layer containingthermoplastic polymeric material and microvoids having a substantiallycircular cross-section in a plane perpendicular to the direction oflight travel and having a diffuse light transmission efficiency of atleast 65%.
 31. The light diffuser of claim 30 wherein the difference inrefractive index between the thermoplastic polymeric material and themicrovoids is greater than 0.2.
 32. The light diffuser of claim 30wherein said microvoids are formed by organic microspheres.
 33. Thelight diffuser of claim 30 wherein said microvoids are substantiallyfree of scattering inorganic particles.
 34. The optical element of claim1 wherein the optical element is comprised of at least two integrallayers, the first layer containing complex polymer lenses and polymercylinders which have a height at least 50 micrometers greater than thecomplex lenses, and the second layer serving as the substrate for thefirst layer.
 35. The optical element of claim 34 wherein the cylindershave a diameter of about 25 micrometers and a frequency of about 20 mm.36. A back lighted device comprising a light source and an opticalelement containing a rough surface having a roughness average equal toat least 5 micrometers wherein the rough surface contains at least tworoughness populations in which the roughness average of the at least twopopulations varies by at least 8 micrometers.
 37. A liquid crystaldevice comprising a light source and an optical element containing arough surface having a roughness average equal to at least 5 micrometerswherein the rough surface contains at least two roughness populations inwhich the roughness average of the at least two populations varies by atleast 8 micrometers wherein the optical element is located between thelight source and a polarizing film.
 38. A method for forming at leasttwo roughness populations in a desired pattern on a transparent supportcomprising the step of coating a melted layer of a polymeric materialonto the support and cooling the material while subjecting the layer tocontact with a form having a roughness pattern corresponding to thenegative of the rough surface of claim
 1. 39. A method for forming atleast two roughness populations in a desired pattern on a transparentsupport comprising continuously casting the polymeric material onto thesupport on a chill roll and cooling the material while subjecting thelayer to a contact with a form having a roughness pattern correspondingto the negative of the rough surface of claim 1.